Control of engine exhaust emissions is becoming increasingly important for engine manufacturers. Governments and regulatory agencies are enforcing ever more stringent emissions standards for many types of on-highway and off-highway vehicles. The amount of pollutants in an exhaust flow emitted from the vehicle's engine may be regulated depending on the type, size, and/or class of engine. Accordingly, manufacturers must develop new technologies to meet these standards while providing high-performance, cost-effective equipment to consumers.
One method implemented by engine manufacturers to comply with the regulation of exhaust pollutants is the use of a selective catalytic reduction (“SCR”) system to clean nitrogen oxides (“NOx”) from the engine exhaust flow. An SCR system works by releasing a reduction agent, such as ammonia (“NH3”), into the engine exhaust flow in the presence of a catalyst. The NH3 may be stored on a surface coating of the catalyst where it reacts with the NOx in the exhaust flow to create environmentally friendly products, such as nitrogen gas (“N2”) and water (“H2O”). The chemical reactions of the SCR process can be represented by:NH3(g)⇄NH3(ads);  (1)4NH3(ads)+4NO+O2→4N2+6H2O;  (2)4NH3(ads)+2NO+2NO2→4N2+6H2O;  (3)8NH3(ads)+6NO2→7N2+12H2O;  (4)4NH3(ads)+3O2→2N2+6H2O.  (5)Reaction (1) describes the ammonia adsorption/desorption from the catalyst, Reactions (2)-(4) are “DeNOx” reactions that describe the reaction between the reduction agent and the NOx in the presence of the catalyst, and Reaction (5) describes the oxidation of the ammonia.
In general, manufactures seek to maximize the amount of NOx in the exhaust flow converted to H2O and N2. To achieve this, the amount of NH3 stored on the catalyst's surface may be increased. However, NH3 may also be desorbed from the catalyst and carried by the exhaust flow downstream of the catalyst to a location where the NH3 is released into the atmosphere. This situation is commonly referred to as NH3 slip. NH3 slip is undesirable because the unreacted NH3 is released into the atmosphere and wasted. The NH3 desorption rate is strongly dependent on the catalyst's temperature. As the temperature of the catalyst increases, the desorption rate of NH3 from the catalyst's surface increases exponentially.
Unlike industrial or stationary SCR applications where engines or turbines generally operate at steady state conditions, mobile SCR systems used for on-highway trucks and off-road machines are subject to transient engine speeds and loads. The transient engine speeds and loads lead to a time varying exhaust temperature, and thus a time varying catalyst desorption rate. For example, a sudden increase in engine load and/or speed may create a sharp increase in the temperature of the exhaust flow. This sharp increase in the exhaust flow temperature may initially heat an inlet portion of the SCR catalyst and significantly increase desorption of the stored reduction agent at the inlet portion. The desorbed reduction agent may be carried downstream and reabsorbed in a cooler downstream portion of the SCR catalyst. As the temperature increase continues to propagate down the length of the SCR catalyst, the reduction agent may correspondingly continue to desorb from the heated portion of the SCR catalyst (the length of which is increasing) and reabsorb in the cooler downstream portion of the SCR catalyst (the length of which is decreasing) until the cooler downstream portion of the SCR catalyst no longer has sufficient capacity to absorb the reduction agent and slip occurs. Automatic control has been used as one method of attempting to handle transient changes in the exhaust flow temperature and desorption rate, while still maintaining a good NOx conversion and avoid slip.
One example of controlling an SCR process is described in SAE paper 2003-01-0776, “Control of a Urea SCR Catalytic Converter System for a Mobile Heavy Duty Diesel Engine” (the '0776 paper) by C. M. Schär et al. Specifically, the '0776 paper discloses a feedforward controller with a surface coverage observer used in conjunction with a feedback controller. The feedforward controller determines an amount of ammonia to be injected using a two-dimensional look-up table and an estimated molar flow of NOx.
The surface coverage observer includes a model of the SCR catalyst and acts as the memory of the feedforward controller. The surface coverage observer includes two cells arranged in series that are used to calculate the surface coverage on an area of the catalytic converter that each cell represents. A maximum surface coverage value for the first cell is determined using a look-up table and is compared to the calculated surface coverage for the first cell. If the calculated surface coverage value is greater than the maximum surface coverage, the surface coverage observer reduces the amount of ammonia to be injected by a given value. The output of the feedforward controller (i.e., the amount of ammonia to be injected) and the output of the feedback controller are then multiplied and sent to the extended plant, which includes the plant itself.
Although the '0776 paper may help reduce slip using a plurality of computational cells, the controller of the '0776 paper may still be suboptimal. For example, a sudden increase in engine load and/or speed may create a sharp increase in the temperature of the exhaust flow and significantly increase desorption of the stored reduction agent. Due to the rapid speed at which the heated exhaust flow may travel, reducing injection of the reduction agent short of completely stopping injection may be insufficient to prevent slip.
Furthermore, the '0776 paper does not disclose how the two cells are oriented relative to the catalytic converter and the exhaust flow direction (e.g., whether they are in series but oriented perpendicular to the flow direction of the exhaust, in series with the first cell near the inlet, in series with the first cell near the outlet, or some other orientation). Moreover, using only two computational cells may be suboptimal. For example, locating the first cell at the outlet of the catalytic converter may lead to overly conservative slip control. By the time that the controller detects a surface coverage at the outlet that is higher than the maximum surface coverage, and commands a reduction in the injected ammonia, slip may have already occurred due to the proximity of the computational cell to the outlet. Conversely, locating the first cell at the inlet of the catalytic converter may lead to overly aggressive slip control since the desorbed ammonia from the inlet computational cell may be reabsorbed into the catalyst downstream of the inlet before it results in slip. This overly aggressive slip control may decrease an NOx conversion efficiency. Similar problems exist for other possible orientations of the two cells. Control systems with only two computational cells also may not have the spatial resolution required to accurately detect the effects of a temperature wave in the catalytic converter.
The present disclosure is directed at overcoming one or more of the problems set forth above.